1. Field of the Invention
The present invention relates generally to methods and apparatus for treating relatively low heating value fuel products, and, more particularly, to an integrated method and system for reducing the moisture content of sub-bituminous coal products, and the like, to produce improved fuel products having heating values comparable to those of bituminous coals.
2. Description of the Prior Art
Since 1975, the production and utilization of sub-bituminous coals (especially those produced in Wyoming's Powder River Basin) has increased dramatically, and today, comprise about 15% of the nation's coal production. Because of the very low sulfur content of these coals (and their resultingly low sulfur dioxide emission potential), it is generally accepted that the production/utilization of sub-bituminous coals will further increase as a result of evolving acid rain legislation, which will require further reductions of sulfur dioxide emissions--especially from the large coal fired electric utility generating stations, that constitute in excess of 80% of the domestic coal demand.
Unfortunately, because of their relatively high moisture content (generally in the range of 30-35%), the `as delivered` heating value of sub-bituminous coals (typically in the range of 8,200 to 8,800 BTU/lb.) is significantly lower than that of bituminous coals (generally in the range of 10,500 to 12,500 BTU/lb.). Because of this lower heating value, many existing coal-fired power stations, especially those which specifically were designed to burn the higher BTU value bituminous coals, are not able to utilize sub-bituminous coals simply because of their lower BTU value which results in the inability to fire sufficient quantities of coal per unit of time to generate the required quantity of heat. This is further compounded by the long shipping distances and high transportation costs from the Wyoming Powder River Basin to the majority of the major coal fired generating facilities (major generating facilities are located near population centers). This significant shipping cost is adversely impacted by the 30% moisture content of the as-mined coal. Therefore, in spite of the forecast increase in potential demand for low sulfur coal and the recognized capability of sub-bituminous coal to satisfy the evolving sulfur emission requirements, the high moisture content of as-mined sub-bituminous coal limits its ability to respond to this opportunity.
In an effort to address this problem, it has become known to thermally dry sub-bituminous coal including using a conventional fluidized bed-type thermal dryer in an attempt to reduce the moisture content, and thereby up-grade the heating value of a sub-bituminous coal product. However, because the moisture in sub-bituminous coal is essentially entirely inherent (i.e., contained within the coal) rather than surface (occurring only on the particle surfaces), the problems and technical requirements associated with the thermal drying of sub-bituminous coal are radically different from those encountered in the thermal drying of bituminous coals, and, for this reason, the direct application of traditional bituminous coal-based thermal drying processes and experience to sub-bituminous coals is not technically appropriate. This fact has been clearly illustrated by the operating experiences of the presently operating sub-bituminous thermal dryer which, as noted above, was designed based on classical bituminous coal thermal drying experience and practice.
An advantage exists, therefore, for a method and system which will successfully, efficiently, and economically thermally dry sub-bituminous materials to raise the heating values of such materials to levels comparable to those of bituminous coals.
It is therefore an object of the present invention to provide a method and system for specifically addressing the unique drying characteristics and requirements of sub-bituminous, lignitic, and similar low-rank coals so as to raise the heating values of such materials to levels comparable to those of bituminous coals.
It is a further object of the invention to provide a method and system for drying sub-bituminous materials which integrates a number of novel and innovative sub-systems and components in order to address the particular drying characteristics and requirements of sub-bituminous materials.
Still other objects and advantages will become apparent in light of the attached drawing figure and written description of the invention presented hereinbelow.